mineral systems: key to exploration targeting

T. Campbell McCuaig and Jon Hronsky1,2,3 1Centre for Exploration Targeting,

2ARC Centre of Excellence for Core to Crust Fluid Systems 3 Western Mining Services

Geological Survey of New South Wales Exploration in the House

May 2016

http://images.publishing.uwa.edu.au/uwa/logos/welcome.asp?choose1=Compact&choose2=Normal&choose3=CMYK

Takeaway Messages Mineral systems are complex dynamic systems exhibiting self-organised critical (SOC) behaviour

Critical elements of mineral systems are whole lithosphere architecture, transient geodynamic triggers, fertility, and preservation of primary depositional zone

In application of the mineral system concept in targeting, SCALE of decision must be matched to scale of relevant geological process

Fertility Favourable

Whole-lithosphere Architecture

Favourable (Transient)

Geodynamics

Ore Genesis

Preservation (of primary depositional zone) +

McCuaig and Hronsky (2014)

The Problem How to predict location and geometry of new high quality mineral districts – camps – oreshoots?

2.7-2.6 Ga Ni and Au in the Yilgarn WA

Craton / district scale

150km

400km

New

Hol

land

Au

(Hen

son,

200

8)

St. I

ves

Au

(Mill

er e

t al.

2010

)

(McC

uaig

et a

l. 20

10)

Deposit scale

Oreshoot scale

100m

2km

Camp scale

LOW

BROAD REGIONAL

PREDICTION

HIGH

PROSPECT SCALE SCALE (km)

REL

ATIV

E EF

FEC

TIVE

NES

S DETECTION

Camp scale decision

1000 100 10 1 0.1

COST

FLEXIBILITY

Alteration halos High definition geophysics Drilling Geochemistry

Where do we focus the more systematic, detailed and expensive detection technologies?

Scale Dependent Targeting

McCuaig et al. (2010)

We Need Non- Traditional Datasets to See Large Footprints

Magnetotelluric Section through Olympic Dam

Modified after Hayward, 2004; Magnetotelluric section provided R. Gill, Uni. Adel; “hotter” colours are more conductive

Areas of Textureless Seismic Response

OD Density Inversion Anomaly

40 km

Moho

Need to understand the entire system

Mineral Systems – Some Constraints

Take elements at low concentration from large volumes of rock to high concentration in small volumes of rock

Only plausible mechanism is through advective mass flux – needs a fluid (fluid/magma)

Ore deposits therefore are foci of large scale advective mass and energy flux

Fluid needs to be low viscosity, available in large quantities over short timeframes, highly organised (focussed in space and time)

Mineral systems are dynamic complex systems

Olympic Dam

Perseverance

Norilsk

Structure and Pattern in earth systems

e.g. Power-law size frequency distributions (scale invariant) in Earth Systems

Understanding Dynamic Complex Systems

Example: Fault size populations

Needham et al., 1996 Malamud & Turcotte 2006

Example: Gutenberg-Richter earthquake scaling.

Example: Superior craton, greenstone-hosted lode gold

Robert et al., 2005

Energy Sink

Energy Source

Potential Energy

Gradient Self-Organized System

Entropy (exported to

environment as diffuse heat)

Energy Flux – fed into system at a slow rate

Energy Flux – Released in transient “Avalanches”

Threshold Barrier

A B

Ore formation as a product of self-organising critical systems






Geodynamics

Ore Genesis


Critical Elements of Mineral Systems


Whole Lithosphere Architecture Isotopic maps as ‘paleogeophysics’ to image paleoarchitecture

Time slices can provide insights into spatial distribution of multiple mineral systems through time PREDICTIVE TOOL

ƐNd Mole et al., 2012 ƐHf Mole et al., 2012

Craton margin, 2.7 Ga Craton margin, 2.9-2.8 Ga

NiS (red), Fe (blue), Au (gold) deposit distributions

NiS deposits (stars) and komatiite Mg# (red) at 2.9Ga

After McCuaig 2003 (courtesy of Antamina); Love et al., 2004

Antamina, Peru

McCuaig and Hronsky, (2014)

Vertical Accretive Growth History – Antamina

Vertical Accretive Growth History Fundamental Lithospheric Flaws


Vertical Accretive Growth History – NSW

Common Characteristics of Large Scale Ore-Controlling Structures Strike-extensive.

Depth-extensive (often lithospheric mantle) with relatively steep dips (as imaged in geophysics).

Commonly juxtapose distinctly different basement domains (as imaged by isotopes and magma chemistry), and differing mineral endowment

Multiply-reactivated (commonly with variable senses of movement) with a very long history.

Vertically-accretive growth histories.

These are not the obvious structures at or above the level of mineralisation – an important message for targeting.




Geodynamics

Ore Genesis




In recent years increasing availability of high-resolution geochronology and better understanding of global geodynamics This is increasingly indicating that major ore-forming events occur in narrow time windows, often over broad areas These critical time horizons must reflect unusual regional-scale geodynamic settings that are favourable for mineralisation These favourable settings must be transient, lasting for only short periods of geological time

Transient Geodynamic Triggers

Focused system with little lateral dispersion

Broad halo but not syn-ore

Transient Geodynamic Triggers Main ore events are transient in a larger magmatic/hydrothermal event


Large Orogenic Au deposit

Large Porphyry Cu-Au deposit

All these major deposits formed at 440 Ma (as did North Kazakhstan Gold Province)

Squire & Miller (2003)

Transient Geodynamic Triggers

Favourable Transient Geodynamic Events Empirically we recognise three common scenarios: Incipient Extension (VMS, Akalic LSE Au, LSE Au, NiS)

Transient Compression (Porphyry Suite deposits, Mafic Intrusion NiS)

Switches in Far-Field Stress (All?)

Favourable Transient Geodynamic Events During these events:

Active permeability creation is stopped, or Vertical permeability is clamped Energy and fluid input to the system continues Extreme energy and fluid pressure gradients are formed The system self-organizes to form ore as long as the geodynamic threshold barrier remains intact.

McCuaig and Hronsky 2014

Threshold Barrier Incipient Extension e.g. VMS-epithermal


Threshold Barrier Transient Anomalous Compression e.g. porphyry

After Rosenbaum et al., 2005

After Sillitoe 2010

Goldfarb et al., 2005

Threshold Barrier Stress Switches e.g. orogenic gold




Geodynamics

Ore Genesis




Fertility

A geological region or time period systematically better-endowed than otherwise equivalent geological environments 4 components

Secular Earth Evolution

Lithosphere fertility (e.g. Au)

Geodynamic context (e.g. magmatic-hydrothermal Cu)

Paleolatitude (Zn-Pb, U, Fe, others?)

Implications of lithosphere enrichment


Fertility - Geodynamic Context

Spreading Rate on the MAR increased rapidly in Cretaceous

This pushed South America hard to

the west

Which made the western margin

anomalously compressional

Andean Cu since the Cretaceous – anomalously compressive margin

Emergence of Ongtong-Java plateau in southern Pacific

This pushed Pacific plate hard

to the ENE


Nested scales of Threshold barriers Transient extreme anomalous compression causes ore

Mineral Systems in Practice

At all scales, processes at site of deposition get heavy weighting, despite their lower relevance to regional targeting decisions Bias is to data rich areas at expense of data poor areas.

Challenge in practical application is in keeping scale of decision matched to scale of relevant mineral system process:

after McCuaig and Hronsky 2000 LOW

BROAD REGIONAL

PREDICTION

HIGH

PROSPECT SCALE SCALE (km)

REL

ATIV

E EF

FEC

TIVE

NES

S DETECTION

Camp scale decision

1000 100 10 1 0.1

COST

FLEXIBILITY

Scale Dependent Targeting

Detect evidence of the mineral system (fertility, whole lithosphere architecture, geodynamics)

REGION

Detect evidence of the zones of mass and energy flux (architecture, local geodynamics, preservation)

CAMP

Find the deposit/oreshoot through systematic detection (alteration, high-res geophys, geochem, drilling)

ORESHOOT DEPOSIT

Scale Dependence of Critical Elements for Orogenic Au


Distal Footprints of Giant Ore Systems

Largest integrated geoscience initiative in Australia

Exploration Incentive Scheme Geology-geophysics integrated 3D models and mineral systems targeting products for industry

And more on the way…

Geophysical investigation of

Kimberley Region, northern Western

Australia

Lindsay et al., in press

Prospectivity of the Halls Creek

orogeny, Western Australia using a mineral systems

approach

Occhipinti et al., in press

Takeaway Messages Mineral systems are complex dynamic systems exhibiting self-organised critical (SOC) behaviour

Critical elements of mineral systems are whole lithosphere architecture, transient geodynamic triggers, fertility, and preservation of primary depositional zone

In application of the mineral system concept in targeting, SCALE of decision must be matched to scale of relevant geological process




Geodynamics

Ore Genesis



A New View on Footprints

In exploration for new high quality mineral districts under cover, it is the largest scale footprint of the deposits that is relevant to our targeting models These large scale footprints differ substantially from the local expressions captured by traditional analogue models

The tendency of complex systems to order around a critical point is termed self-organised criticality (SOC; Bak et al 1987) Key drivers of SOC behaviour are:

Energy is added slowly over long timeframes A barrier (threshold barrier) to energy flux is present that stops dissipation into the energy sink, forming extreme energy gradients Energy is released over very short timeframes in dramatic pulses termed ‘avalanches’

These systems will remain SOC systems as long as the energy flow is maintained, and the threshold barrier is intact.

A new understanding of physics of complex systems


Whole Lithosphere Architecture

A Multiscale Fluid (incl Magma) Delivery System

McCuaig and Hronsky, 2013

Retreating arc Advancing arc

Small volume melts trapped in mantle lithosphere

Au transferred to crust by subsequent tectonic and thermal trigger

Hronsky et al., 2012

Fertility – Lithosphere Enrichment

mineral systems: key to exploration targeting

Science